1. Technical Field
The present invention relates to an apparatus for performing a work operation on a sheet material while driving the sheet material along a feed path and, more particularly, to a plurality of drive wheels that engage the sheet material for driving the sheet material along the feed path therein.
2. Background Art
Friction, grit, or grid drive systems for moving strips or webs of sheet material longitudinally back and forth along a feed path through a plotting, printing, or cutting device are well known in the art. In such drive systems, friction (or grit or grid) wheels are placed on one side of the strip of sheet material (generally vinyl or paper) and pinch rollers, of rubber or other flexible material, are placed on the other side of the strip, with spring pressure urging the pinch rollers and material against the friction wheels. During plotting, printing, or cutting, the sheet material is driven back and forth, in the longitudinal or X-direction, by the friction wheels, while, at the same time, a pen, printing head, or cutting blade is driven over the sheet material in the lateral or Y-direction.
These systems have gained substantial favor because of their ability to accept plain (unperforated) strips of material in differing widths. The drive wheels play a paramount role in moving the sheet material along the feed path. Each wheel is typically associated with a longitudinal edge of the sheet material and has a cylindrical shape with a plurality of teeth formed on the surface thereof. The teeth engage and drive the sheet material along the feed path. The tooth pattern of the drive wheels is embossed on the back of the sheet material as the sheet material is driven through the friction feed apparatus with the pinch rollers urging the sheet material against the drive wheels. Ideally, as the direction of feed of the sheet material is reversed in the friction drive apparatus during a work operation, the teeth fit into the embossed marks that were previously formed during the forward feed. The wheels must engage the sheet material to prevent any slippage of the sheet material because even relatively minor slippage can have a detrimental effect on the work operation being performed by the apparatus. Also, the drive wheels must engage the sheet material without causing visible damage to the sheet material. Additionally, the teeth must not be brittle to avoid breakage and wear during the work operation.
One existing type of friction wheel is fabricated by first, chemically etching a tooth pattern on a flat sheet of material. Then, the etched material is cut into strips and the strips are helically wrapped around a donor hub and welded at the seams to form a cylindrical shape. Subsequently, a special coating is applied. The cylinder is then removed from the donor hub and fitted onto a finished hub to form a grit wheel. First, the fabrication process for this type of a friction wheel is time consuming and expensive. Second, the process does not yield consistent results. Since the friction wheels for each apparatus have to be well matched, a subsequent inspection process is required. Therefore, this type of friction wheel is not very desirable.
Another type of friction wheel used in the industry has a cylindrical shape with a gritty circumferential surface and a random pattern. The gritty surface is formed by mechanically bonding tiny solid particles, such as diamond dust or chips. However, in this process it is difficult to ensure that the outside and drive diameters of one friction wheel are substantially identical to the outside and drive diameters of another friction wheel. The drive diameter of the friction wheel is critical because it determines how much the sheet material advances in the longitudinal direction. If two wheels have different drive diameters, one side of the sheet material will be driven greater distance in the longitudinal direction than the other side of the sheet material. Thus, the sheet material will potentially skew in the apparatus and result in damaged graphic image. This becomes especially problematic for longer graphic images and higher speed apparatus, wherein even a small difference in the drive diameter of the wheels results in large errors.
Yet another type of a friction wheel that is used in the industry has a knurled surface. The knurled surface is formed by displacing material to form an irregular pattern. The knurling process also does not yield substantially identical friction wheels.
A U.S. Pat. No. 4,903,045 entitled "X-Y Plotter For Non-Perforated Paper" to Sakamoto et al. discloses a drive roller with a cylindrical reference surface having a plurality of projections of sharp quadrangular pyramidal shape distributed over a reference surface. The sharp projections perforate the paper as the paper is driven through the plotter. The reference surface radially supports the non-thrusted portions of the paper and drives the paper by the friction force. The disclosed drive wheels have several deficiencies and drawbacks. First, the sharp projections perforate the paper and damage the graphic image. These drive wheels are not suitable for wider paper that requires additional wheels in the middle of the plot. Additionally, the sharp projections over time become dull and therefore cause reduction in driving force.
Another U.S. Pat. No. 4,683,480 entitled "X-Y Plotter Drive Roller Arrangement" to Sakamoto et al. also discloses a drive roller with a plurality of sharp projections that perforate the paper. The sharp projections are disposed substantially adjacent to each other thereby not providing sufficient driving force and allowing slippage of the sheet material.
Therefore, it is desirable to provide a drive roller that meets requirements and overcomes the deficiencies described above.